1. Field of the Invention
This invention relates generally to high security, tamper resistant contact systems and more particularly to a logic circuit including magnetically actuated sensors that are resistant to foreign magnetic fields and that do not require custom tuning for monitoring door or window openings or the like as part of an electrically monitored physical security system.
2. Description of the Background Art
Thieves and intruders of increasing sophistication continue to challenge the peaceful security of businesses and homes alike. Because security personnel cannot be present at all potential points of entry to a structure, effective security systems require accurate indicators of the movement of doors and windows and other points of entry. Similarly, safety control systems on industrial machinery require accurate indicators as to whether a safety guard or cover is in place before activating or deactivating the machine.
Magnetically actuated proximity switches known in the art typically have one or more reed switches mounted to a frame surrounding a window or doorway that are electrically connected to the security control unit. One or more permanent magnets are mounted to a door or window in predetermined positions. When the permanent actuating magnet is brought in proximity to the reed switch, as determined by the sensitivity of the reed switch and the strength of the permanent magnet, the reed switch is actuated by the permanent magnet by closing a set of magnetic contacts within the switch and completing the circuit. A signal is thereby sent to the control unit indicating that the door is in proper position.
One deficiency with conventional magnetic proximity switches is that the permanent actuating magnets must be precisely positioned with respect to the reed switch, thereby requiring periodic adjustments to avoid false alarms or an ineffective system. Consequently, regular adjustments must be made as a result of seasonal changes in temperature and humidity, as well as frequent use of the door or window that may lead to misalignment of the permanent magnet and switch. Misalignment may result in false alarms as well as unscheduled service calls for manual realignment of the switch or magnet.
It is therefore desirable to use a fairly sensitive switch so that the device has a greater tolerance to small deviations in the position of the permanent actuating magnets and the switch without setting off a false alarm. Sensitivity of the reed switches has been improved in the art by the placement of small biasing magnets near the reed switch to bias the response of the switch to external magnetic fields. The biasing magnet may be oriented in polar opposition to the actuating magnet thereby increasing the sensitivity of the reed switch. Thus, smaller and less expensive magnets may be used as actuating magnets.
Another deficiency of conventional magnetically actuated proximity switches is that they may be subject to circumvention or manipulation by strong foreign magnetic fields. For example, a conventional magnetic switch can be defeated by the placement of an external magnet near the switch. A magnet may be used to defeat a conventional magnetic switch on the opposite side of the door if it produces a sufficient magnetic field. The intruder can open the door without activating the alarm because the strong external magnetic field caused the switch to stay in the same state as when the actuating magnets are in the proper position.
Later “balanced” type switches were developed that may be sensitive to externally applied magnetic fields. One approach to solving the aforementioned deficiencies can found in U.S. Pat. No. 4,945,340 to Brill, incorporated herein by reference. The Brill patent discloses an apparatus comprising three switches, two of which are responsively adapted to a fixed magnetic field placed in proximity to the switches. The third switch is positioned to detect when another magnetic field, introduced by someone who is attempting to defeat the security system, is placed in proximity to the three switches and the fixed magnetic field. One apparent deficiency of the Brill approach, however, is that the third switch fails to detect when another magnetic field of the same form produced by the magnet pack assembly described therein is placed in proximity to the three switches. Therefore, Brill's approach is easily compromised by anyone placing another like fixed magnetic field in proximity to the three switches and thereby disabling the security system.
Magnetically biased high security switches also have disadvantages due to changes in the magnetic strength of the biasing magnets. For example, due to their inherent sensitivity, they may malfunction and cause false alarms. Accordingly, the switches and magnets must be carefully adjusted and positioned during installation to avoid false alarms. Furthermore, magnetically balanced switches are difficult to manufacture and are costly because the magnets must be magnetically balanced very carefully, either during installation, or preset at the factory.
Therefore there is a need for a switch apparatus that cannot be compromised by the placement of an additional magnetic field placed in proximity to the apparatus for the purpose of defeating the security system. There is also a need for an apparatus that will detect the presence of an additional magnetic field in proximity to the apparatus. There is also a need for a switch apparatus that will enter into an alarm state when an additional magnetic field is placed in proximity to the apparatus. There is also a need for a switch apparatus that can be precisely assembled and obviates the need to perform adjustments at the factory. The present invention satisfies these needs, as well as others, and generally overcomes the deficiencies found in existing equipment.